I should have clarified the GUI newsreaders. I'd be willing to bet a very small amount of money that around 2000 the majority of newsreaders in use by real humans (not spam bots) were Outlook Express and Netscape.
There were some good GUI newsreaders but Outlook Express was a shit show. I remember OE was what my ISP had NNTP documentation for and nothing else.
> Thunderbird was still pretty popular around then as well
Thunderbird didn't come out until the early 2000s. The predecessors were Mozilla mail & news and Netscape Communicator. Seamonkey[1] is an offshoot of the former once Mozilla split mail and news into Firefox and Thunderbird.
There were some good GUI newsreaders but Outlook Express was a shit show. I remember OE was what my ISP had NNTP documentation for and nothing else.